1. Related Application
The subject matter of the present application is related to the subject matter of my co-pending application Ser. No. 189,595, filed on Sept. 22, 1980 and assigned to the same assignee as the present application, the teachings of which are incorporated herein by reference. My earlier application corresponds to German OS No. 29 41 452.
2. Field of the Invention
The present invention relates to methods for encoding analog signals for storage or transmission purposes, and in particular to such methods wherein the sampled momentary values of the analog signals are converted into PCM code words in an encoder and PCM differential code words (.DELTA.PCM words) are subsequently formed from two successive PCM words by means of differential formation and wherein each of the .DELTA.PCM words representing a pulse group, together with at least one auxiliary code element, form a code word of a prescribed length corresponding to the length of a reference code word.
As described in my co-pending application Ser. No. 189,595, uniformly quantized pulse code modulation negatively effects the quality of encoded analog signals which are stored or transmitted. In order to reduce the number of elements of the code words, so-called differential pulse code modulation can be used in which only the amplitude changes of successive momentary values of the analog signal are stored or transmitted in digitally encoded form. Within specified quality demands, the number of code elements thereby arising per .DELTA.PCM word is determined by the maximum possible change of the amplitude between two successive momentary values of the analog signal. This permits the possibility of reducing the transmission or storage capacity of the transmitting or storage medium for the reason that the number of code elements in a code word is selected only as large as is respectively necessary for the representation of the particular momentary value. Such reduction, however, requires additional code elements in each word which, given differing word length, can be lost due to bit errors, resulting in errors when the original analog signal is regained. For this reason PCM redundancy reduction methods usually employ a constant number of code elements for the respective transmission of the momentary signal values, the number of code elements being reduced in comparison to a maximum value which would exist for uniform quantization. The binary encoded momentary values, identified by means of appropriate additional code elements, are transmitted, either individually or in blocks, in companded form. This method, however, results in the undesirable dependency of the quality of the regained analog signal on the degree of companding.
In order to avoid the undesirable dependency of the quality of the regained analog signal on the degree of companding for the purpose of transmitting or storing digitally encoded analog signals even given the use of a reduced constant code element group per code word for each momentary value, the method disclosed in my earlier application Ser. No. 189,595 further reduces the digital signal flow by selecting the reference code word, including the auxiliary code elements, to be shorter than the maximum possible length of a .DELTA.PCM word and continuously compares the length of the generated .DELTA.PCM words to the reference code word. Only upon the detection of a .DELTA.PCM word of excess length is the excess-length word sufficiently abbreviated by means of either eliminating some of the least significant code elements, thereby forming a companded .DELTA.PCM word, or by replacing the excess-length word by a PCM word of suitable length.
The PCM word of suitable length in the method of my earlier application is formed either by eliminating the least significant code elements or by dividing the code elements into two successive code words, forming a PCM double code word, which respectively comprise the least significant and most significant bits of the excess-length word.
Because large amplitude changes occur with limited and irregular distribution in analog signals, such as voice and music signals, conversion of the excess-length .DELTA.PCM words into PCM words of suitable lengths has substantially no effect in reducing the quality of the regained original analog signal. Disadvantages may occur, however, if a continuous transmission of such a reduced digital signal flow is disrupted by, for example, brief interruptions of the signal flow on the transmission path. Because essentially only signal differences in the form of .DELTA.PCM words are transmitted, each signal flow interruption effects an error propagation in the subsequent signal flow which can only be corrected when a PCM double word is transmitted which, in effect, represents a signal reference value.